• bessel(n, scale)—Returns coefficients for an analog Bessel filter of order n; scale controls the gain at cutoff. The output is used as an argument to one of the IIR filter functions.
• butter(n)—Returns coefficients for an analog Butterworth filter of order n. The output is used as an argument to one of the IIR filter functions.
• cheby1(n, e)—Returns coefficients for a Type I Chebyshev analog filter of order n; ripple is controlled by the parameter e. The output is used as an argument to one of the IIR filter functions.
• cheby2(n, scale, atten)—Returns coefficients for a Type II Chebyshev analog filter of order n with the lower edge of the stopband at scale and stopband attenuation atten. The output is an argument to the IIR filter functions.
Arguments
• n is an integer filter order:
◦ For bessel filters, n is between 1 and 35
◦ For all others, n is between 1 and 200.
• e is a positive real number representing the allowable passband ripple.
• scale is a calculated real number greater than 1 setting the lower edge of the stopband:
◦ For bessel filters, it is a real number greater than 0 which can be used to control the gain at the cuttoff frequency.
◦ For cheby2 filters, it is a calculated real number greater than 1 setting the lower edge of the stopband.
• atten is a real number greater than 1, the stopband attenuation factor.
Additional Information
These functions produce coefficient arrays with an even number of columns:
• If there are two columns, as in the output of bessel, the filter consists of a single section: column 1 contains coefficients of the transfer function numerator, column 2 contains denominator coefficients.
• If there are more than two columns, each pair represents a section of a filter in cascade form.